What is the minimal amount of disk space needed to set up a nominatim server/db for just the United States + PR?

I’m trying to set up a nominatim server/database for United States + Puerto Rico. My constraint is that I only have 100GB of available disk space. Is it possible to set up a nominatim server with that little space? I tried using the latest usa osm file from GeoFabrik, but it used up all my available memory.

Would it help if I import an osm file with only zip codes? Are there ways to greatly reduce the imports?

Interpolating from what is needed for the whole planet, I recommend that you should have at least 300GB of disk available. Some of this can be freed when the import is done.

Import - Nominatim 4.4.0 Manual has a few hints on how to reduce the size of the database. In particular, have a look at the import styles. If you adapt the style to only import what you need, it might be possible to reduce the required space. Getting down to 100GB is going to be tough.